Optical add/drop wavelength division multiplex systems

ABSTRACT

A method of, and system for, controlling one or more wavelength channels added to an optical fiber trunk of a WDM system from a branch terminal via a saturated amplifier in which an additional wavelength signal different from the wavelength of wanted channels is introduced to the input of the saturated amplifier to reduce the level of the added wanted channel(s).

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to improvements in optical add/drop wavelengthdivision multiplex (WDM) systems and more particularly to the control ofadd channel level in such systems. the invention has particularapplication in underwater cable systems employing optical fibre cables.

In WDM transmission systems several wavelengths are transmitted along amain trunk which is provided with one or more branching units each ofwhich is arranged to route one or more particular wavelength channels toan individual branch terminal, so called drop channels. The branchterminal is arranged to add one or more channels of the same ordifferent wavelengths to the trunk. Such systems require the control ofpower levels of the added channels under normal operating and faultconditions to ensure that individual channels are not excessivelyimpaired compared to other channels. In the case of branches whichinclude saturated optical repeater amplifiers which introduce severaldBs of compression the delicate balance of optical powers in individualchannels can easily be upset or irrevocably impaired. The presentinvention seeks to provide simple adjustment or control of the amplitudeof add channels provided by such an amplifier.

BRIEF SUMMARY OF INVENTION

According to the invention there is provided a method of, and systemfor, controlling the level of one or more wavelength channels added toan optical fibre trunk of a WDM system from a branch terminal via asaturated amplifier in which an additional wavelength signal differedfrom the wavelength of wanted channels is introduced to the input of thesaturated amplifier to reduce the level of the added wanted channel(s).

Preferably the additional wavelength signal is variable in level, orstep adjustable, to permit control of the level of the added signal(s).

Preferably the additional wavelength signal is supplied from the branchterminal.

BRIEF DESCRIPTION OF DRAWING

FIG. 1 is a schematic view of an add/drop wavelength division multiplexsystem according to the invention.

DETAILED DESCRIPTION OF INVENTION

The single drawing FIG. 1, illustrates schematically by way of exampleonly, a method and system in accordance with the invention and, forclarity of description, only a single fibre and single branch is shownalthough it will be appreciated that the invention is applicable tomultiple fibre and/or multiple branch systems.

A main trunk 10 has an optical amplifier or repeater 12, and a branchingunit 14. The branching unit has a first three port circulator 16connected in series with a filter 18 and a second three port circulator20. One port of the first circulator is coupled to a branch terminal 22via a drop channel including an optical amplifier or repeater 24 and anoptional filter 26. The other circulator 20 has one port connected to anadd channel including an optical amplifier 28 to the branch terminal 22.The amplifier 28 is a saturated amplifier and the maximum poweravailable is shared between the various different channel wavelengths tobe added to the trunk. By providing an additional optical wavelength ata frequency not required for transmission of information on the trunk asan input to the amplifier 28, the output level of the other wantedwavelength channels added to the trunk are reduced. The additionaloptical wavelength may be such that it will pass back through the filter18 to the first circulator 16 and along the drop channel. The dropchannel may include a filter 26 or 26 a to prevent passage of theadditional wavelength. Such a filter 26 b may alternatively be providedin the add channel between the amplifier 28 and second circulator 20.

The additional optical wavelength may be provided by the branchterminal. Such an optical wavelength may be provided by an opticalcontrol signal generator 30 in the branch terminal 22 or at some otherpoint in the system that permits routing of the signal to the input ofthe amplifier. The additional optical wavelength may be variable inamplitude or adjustable in steps to control the level of the channelsadded to the trunk. Such variation may be effected by means of anamplitude controller or attenuator 32 in the branch terminal.

The input to the optical amplifier at A has several data channels atdifferent wavelengths. The signal at output B is split so that one ormore channels is diverted to a different destination. At point C areturn signal from a second destination i.e. branch terminal 22 isreinserted onto the main signal path. At point C it is important thatthe injected signal level is controlled so that there is no degradationof the signals passing through the main path.

With an unrepeatered drop path then the add level may be controlleddirectly from the terminal. With a repeatered drop path then control ofthe add signal is more difficult because prior to the present inventionthere was no simple way by which the signal level could be controlledfrom the terminal because of the self regulation of the amplifiers whichare in compression.

This invention relates to a method of, and system for controlling theadd signal level provided by one or more amplifiers between the terminaland the add point C of the system.

Looking at an application of the invention from another viewpoint, inrepeatered WDM systems the control of added signals to the trunk iscritical to ensure that the pre-emphasis levels are preserved tominimise the system penalties. This can be carried out using acontrolled gain amplifier prior to the injection point. The proposedmethod controls the level of the added signal by sending a level controlsignal from, for example, the terminal at the start of the system suchthat it takes some of the output power of the injection amplifier. Byvarying the pre-emphasis level of the control signal at the terminal theoutput level of the added signal may be varied to optimise thepre-emphasis at the injection point.

By employing, the invention, simple control of the added signal level atthe injection point may be effected without the need to be able tocontrol the gain of the injection amplifier remotely via a supervisorycontrol channel thus making the design of the amplifier very simple.

What is claimed is:
 1. A wavelength division multiplex opticaltransmission system comprising: (a) a main trunk; (b) a branching unitarranged to drop one or more wavelength channels from the main trunk viaa drop channel and to add an added signal of one or more wanted addwavelength channels to the main trunk via an add channel; (c) a branchterminal providing the one or more wanted add wavelength channels to theadd channel; (d) a saturated amplifier connected in the add channelbetween the branch terminal and the branching unit and having the wantedadd channel wavelengths on the add channel as an input; and (e) meansfor generating an additional optical wavelength signal and providing theadditional optical wavelength signal as an input to the saturatedamplifier, the additional optical wavelength signal having a wavelengthdifferent from the wanted add wavelength channels, thereby reducing thelevel of the wanted add wavelength channels provided to the branchingunit on the add channel.
 2. A wavelength division multiplex opticaltransmission system as claimed in claim 1, wherein the means forgenerating an additional optical wavelength signal is an optical controlsignal generator provided in the branch terminal.
 3. A wavelengthdivision multiplex optical transmission system as claimed in claim 1 andfurther comprising means for varying the level of the additional opticalwavelength signal provided as an input to the saturated amplifier.
 4. Awavelength division multiplex optical transmission system as claimed inclaim 3, wherein the means for varying the additional optical wavelengthsignal is adjustable in steps.
 5. A wavelength division multiplexoptical transmission system as claimed in claim 1 and further comprisinga filter provider in the drop channel to prevent passage of theadditional optical wavelength signal.
 6. A wavelength division multiplexoptical transmission system as claimed in claim 5, wherein the dropchannel further includes an amplifier and the filter is provided priorto the amplifier in the drop channel.
 7. A wavelength division multiplexoptical transmission system as claimed in claim 1 and further comprisinga filter provided in the add channel between the saturated amplifier andthe branching unit, to prevent passage of the additional opticalwavelength signal.
 8. A wavelength division multiplex opticaltransmission system as claimed in claim 1 and further comprising afilter provider in the add channel between the saturated amplifier andthe main trunk, to prevent passage of the additional optical wavelengthsignal.
 9. A wavelength division multiplex optical transmission systemas claimed in claim 1, wherein the branching unit comprises first andsecond three port optical circulators, the first three port opticalcirculator having a first port connected to the main trunk for receivingincoming signals, a second port coupled via a filter to a first port ofthe second optical circulator, and a third port coupled to the dropchannel, the second three port optical circulator having a second portcoupled to the main trunk for transmitting outgoing signals and a thirdport is coupled to the add channel.
 10. A method of controlling thelevel of one or more wanted add wavelength channels added to an opticalfiber trunk of a wavelength division multiplex optical transmissionsystem comprising: (a) providing the one or more wanted add wavelengthchannels to the input of a saturated amplifier; (b) providing anadditional optical wavelength signal to the input of the saturatedamplifier at a wavelength different from the wavelengths of the one ormore wanted add wavelength channels; (c) providing the output of thesaturated amplifier on an add channel to the optical fiber trunk. 11.The method claimed in claim 10 wherein the one or more wanted addwavelength channels are provided from a branch terminal.
 12. A method asclaimed in claim 10, and further comprising varying the level of theadditional optical wavelength signal to adjust the level of the one ormore wanted add wavelength channel on the output of the saturatedamplifier.
 13. A method as claimed in claim 12, comprising varying thelevel of the additional optical wavelength signal in steps.
 14. A methodas claimed in claim 10, and further comprising filtering the output ofthe saturated amplifier provided to the channel to prevent passage ofthe additional optical wavelength signal to the optical fiber trunk. 15.A method as claimed in claim 10, and further comprising filtering a dropchannel of the wavelength division multiplex optical transmission systemto prevent passage of the additional optical wavelength signal from theoptical fiber trunk to a branch terminal connected to the drop channel.16. A wavelength division multiplex optical transmission systemcomprising: (a) a main trunk; (b) a branching unit interposed in themain trunk, the branching unit having first and second circulators eachhaving first, second and third ports, the trunk connected to the firstport of the first circulator, the second port of the first circulatorconnected to the first port of the second circulator, and the secondport of the second circulator connected to the main trunk for passingwavelength channels along the main trunk; (c) a drop channel connectingthe third port of the first circulator to a branch terminal fortransferring one or more wavelength channels from the main trunk, thedrop channel including an amplifier; (d) an add channel connecting thebranch terminal to the third port of the second circulator fortransferring one or more wanted add wavelength channels to the maintrunk, the add channel including a saturated amplifier; (e) an opticalcontrol signal generator in the branch terminal for generating anadditional optical wavelength signal on the add channel at a wavelengthdifferent from the wavelengths of the one or more wanted add wavelengthchannels, the additional optical wavelength signal thereby provided asan input to the saturated amplifier and adjusting the level of the oneor more wanted add wavelength channels; and (f) at least one filter forremoving the additional optical wavelength signal from the wavelengthdivision multiplex optical transmission system after adjustment of thelevel of the one or more wanted add wavelength channels.
 17. Awavelength division multiplex optical transmission system as claimed inclaim 10, wherein the filter is in the add channel between the saturatedamplifier and the third port of the second circulator.
 18. A wavelengthdivision multiplex optical transmission system as claimed in claim 10,wherein the filter is in the drop channel between the third port of thefirst circulator and the amplifier in the drop channel.